Lenovo T490 flashing power LED

[skyblaster]

I have a T490 that will not power on by pressing the power button. If I short out the pins on the ribbon cable however, I can successfully get it to boot on occasion.

When it is on however, the LED flickers: https://www.youtube.com/watch?v=d6A4hnH3d_Q

As there are not many components on the power board itself, how likely is it that replacing it will fix this issue?

I lack the necessary equipment for BGA repair, so really hope it's not the MEC1663 (UEC1) IC.

[skyblaster]

Does anyone have any ideas from the LED flickering?

Here is what I measure on EMC@D5101 AZ5125-02S.R7G_SOT23-3 while the system is running.
1-2 = 1.97V
1-3 = 0.00V
2-3 = 1.97V

[skyblaster]

Correction to the previous post.
1 - GND
2 - -LED_PWR_R5101
3 - -PWRSWITCH

With a fully charged battery and the charger connected, but system in the off state, voltages are as follows:
1-2 = 3.17V
1-3 = 0.51V
2-3 = -2.67V

In the powered on state (charger disconnected) I see:
1-2 = 0.00V
1-3 = 1.97V
2-3 = 1.97V

[mon2]

D5101 is a TVS diode. This part will be passive unless there is a high voltage transient event at which point this part will sacrifice itself and shunt the voltage to ground. After this transient event, the protected line will appear to be a short to ground and respectively not function as designed. Then this TVS part must be removed and/or replaced.

[skyblaster]

Yes. I knew to look for that IC after reading some of your posts ;-)
I don't detect any shorts on the TVS, so I guess I should probably purchase a new NS-B902 power button board, as I'm out of ideas.

I did find a tarnished pin on my 65W Lenovo power supply and it would not always light up the side mounted power led when inserted into the laptop, so I'm now using an equivalent Dell supply without issue.

After a full disassemble/reassemble, the power button LED was solid for several boots, but is now back to off with the odd dim flash. Luckily the button will boot the machine more often than not after reassembly.